tpo52三篇托福阅读TOEFL原文译文题目答案译文背景知识.docx

PAGE

PAGE1

tpo52三篇托福阅读TOEFL原文译文题目答案译文背景知识

TOC\o1-2\h\u7783阅读-1 2

32469原文 2

16042译文 5

7423题目 7

9460答案 14

28394背景知识 18

19314阅读-2 23

9932原文 23

8540译文 26

11652题目 28

28784答案 37

8375背景知识 40

22324阅读-3 42

1652原文 42

18041译文 45

2889题目 48

9135答案 56

1457背景知识 60

阅读-1

原文

StreamDeposit

①Alarge,swiftstreamorrivercancarryallsizesofparticles,fromclaytoboulders.Whenthecurrentslowsdown,itscompetence(howmuchitcancarry)decreasesandthestreamdepositsthelargestparticlesinthestreambed.Ifcurrentvelocitycontinuestodecrease-asafloodwanes,forexample-finerparticlessettleoutontopofthelargeones.Thus,astreamsortsitssedimentaccordingtosize.Awaningfloodmightdepositalayerofgravel,overlainbysandandfinallytoppedbysiltandclay.Streamsalsosortsedimentinthedownstreamdirection.Manymountainstreamsarechokedwithbouldersandcobbles,butfardownstream,theirdeltasarecomposedmainlyoffinesiltandclay.Thisdownstreamsortingiscuriousbecausestreamvelocitygenerallyincreasesinthedownstreamdirection.Competenceincreaseswithvelocity,soarivershouldbeabletotransportlargerparticlesthanitstributariescarry.Oneexplanationfordownstreamsortingisthatabrasionwearsawaythebouldersandcobblestosandandsiltasthesedimentmovesdownstreamovertheyears.Thus,onlythefinesedimentreachesthelowerpartsofmostrivers.

②Astreamdepositsitssedimentinthreeenvironments:Alluvialfansanddeltasformwherestreamgradient(angleofincline)suddenlydecreasesasastreamentersaflatplain,alake,orthesea;floodplaindepositsaccumulateonafloodplainadjacenttothestreamchannel;andchanneldepositsforminthestreamchannelitself.Bars,whichareelongatedmoundsofsediment,aretransientfeaturesthatforminthestreamchannelandonthebanks.Theycommonlyforminoneyearanderodethe